Wafer protective sheet

ABSTRACT

A wafer protective sheet  1  is made of a synthetic resin sheet with a thickness of 80 to 130 μm having a large number of projected parts and recessed parts on its front and rear surfaces. The large number of projected parts and the large number of recessed parts are respectively disposed so that each part is positioned at intersections of lattice stripes, and the projected part and the recessed part are disposed alternately each other. The sheet has such a water-like cross section that the recessed parts in the rear surface match the projected parts on the front surface and the projected parts on the rear surface match the recessed parts in the front surface. The wafer protective sheet  1  has a bending resistance of 30 to 80 mm. The wafer protective sheet  1  of the present invention is sufficiently thin. When the wafer protective sheets are interposed between stacked wafers to protect them, the sheets do not adhere to the wafers. Thus, wafers housed in a container can be protected against vibration during transportation.

TECHNICAL FIELD

The present invention relates to a wafer protective sheet for protectingsemiconductor wafers that are stacked on top of one another in acontainer.

BACKGROUND ART

A semiconductor wafer (hereinafter referred to as a wafer) is generallya thin disk cut out of a silicon single-crystal ingot, having a diameterof, for example, about 2 to 12 inches. The wafer is provided with manycircuits, such as those for LSI's, on its surface and is then cut intochips. The chips are packaged, thus producing semiconductor devices,such as LSI's.

In the above process, the steps of cutting wafers out of the ingot,providing circuits on the wafers, and cutting the wafers into chips andpackaging them are often performed in different places. For thetransportation of the wafers from a place where one of theabove-mentioned steps is carried out to another place, a specialcontainer of the wafers which is manufactured for such transportation isgenerally used.

For example, the wafers are stacked on top of one another and housed inthe container. Unfortunately, the stacked wafers are liable to getscratched by their rubbing against each other. It is, accordingly,necessary to interpose wafer protective sheets made of, for example,polyethylene, between the wafers to prevent scratches.

Nowadays, in general, housing wafers in and taking them out of thecontainer are performed by an automatic machine. For example, a pick-uparm having a vacuum suction portion is used for taking wafers out of acontainer. An optical sensor provided at the vicinity of the vacuumsuction portion of the pick-up arm detects a position where wafers areplaced and further distinguishes between the wafers and the protectivesheets, and the pick-up arm thus transfers the wafers and the protectivesheets to their respective places. In that case, since the front andrear surfaces of the wafer are mirror-finished, the wafer protectivesheets are adhered to wafers which sometimes causes the automaticmachine to be out of order. In order to prevent such a problem, anapproach has been proposed in which the wafer protective sheet isprovided with projected parts and recessed parts on its surface (referto, for example, Japanese Patent Application Laid-open No. Hei9-129719).

In a container in which wafers are housed, pressure is applied to thewafers in their thickness direction by providing cushioning material,etc. at the bottom and top of the inside of the container and coveringthe container, so that the wafers are prevented from being moved andthus damaged by vibration during transportation. In this instance, ifthe wafer protective sheets are hard, the projected parts and recessedparts of the protective sheets easily make scratches on the wafers. Inorder to prevent scratches, a flexible wafer protective sheet made of anonwoven cloth has been proposed. However, fibers from such waferprotective sheets may negatively affect the wafers due to drop-out offibers from the wafer protective sheet. It may be considered to arrangea wafer protective sheet to be laminated body and to form its surface byusing a soft material. However, this results in high cost,disadvantageously.

Wafers must be free from dust or foreign matter and therefore, waferprotective sheets are hardly reused. In view of disposal, a waferprotective sheet is desired which has a thickness as thin as possible soas to produce few amount of waste.

Accordingly, an object of the present invention is to provide a waferprotective sheet a plurality of which are interposed between wafersstacked on top of one another so as to protect the wafers, which doesnot adhere to the wafers so as to be easily handled by an automaticmachine, which can protect the wafers contained in a container fromvibration during transportation, and which is made of a thin materialfrom the viewpoint of disposal.

DISCLOSURE OF INVENTION

In order to accomplish the above object, the inventors of the presentinvention have conducted research for characteristics required of waferprotective sheets. As a result, the inventors found that a type of waferprotective sheet can prevent wafers from being scratched and that itdoes not adhere to wafers and is accordingly easy to be handled by anautomatic machine. This type of wafer protective sheet has a largenumber of alternately arrayed projected parts and recessed parts, andhas such an appropriate bending resistance as the wafer protective sheetturns flat or substantially flat with the projected parts and recessedparts pressed while a pressure is being applied to stacked wafers in acontainer in their thickness direction, and as the projected parts andrecessed parts are restored to the original state when the container isuncovered to remove the pressure. The inventors also found that thewafer protective sheet can be made of a synthetic resin sheet having abending resistance and satisfying characteristics required of the waferprotective sheet even if its thickness is reduced to 130 μm or less,preferably 110 μm or less. Thus, the present invention was accomplished.

The present invention is directed to a wafer protective sheet made of asynthetic resin sheet with a thickness of 80 to 130 μm having a largenumber of projected parts and a large number of recessed parts on therespective front surface and rear surfaces thereof. The large number ofprojected parts and the large number of recessed parts are respectivelydisposed in such a manner that each part is positioned at intersectionsof lattice stripes, and the projected part and the recessed part aredisposed alternately each other. The sheet has such a wavy cross sectionthat the recessed parts in the rear surface match the projected parts onthe front surface and the projected parts on the rear surface match therecessed parts in the front surface. Also, the sheet has a bendingresistance of 30 to 80 mm.

Since the wafer protective sheet of the present invention is made of asynthetic resin with a thickness of 80 to 130 μm whose bendingresistance has been set at 30 to 80 mm by providing the projected partsand recessed parts, the wafer protective sheet has such a flexibility asprojected parts and recessed parts turns flat or substantially flat whena pressure of, for example, 1×10³ to 1×10⁴ Pa, is applied to wafers in acontainer in the thickness direction of the wafers so that the wafers donot move. Thus, the possibility of making scratches on the wafers isreduced, and adhesion can be prevented effectively because the projectedparts and recessed parts are restored to the original state when thecontainer is opened.

In addition, in the wafer protective sheet of the present invention, thelarge number of projected parts and recessed parts are alternatelyarrayed in such a manner as to be positioned at intersections of latticestripes. Consequently, the bending resistances in the lengthwise andcrosswise directions of the wafer protective sheet are substantially thesame; hence stiffness of the sheet has no orientation. Furthermore,since the projected parts are formed on both the front surface and therear surface so that the resulting sheet has no distinction between thefront surface and the rear surface, the wafer protective sheet can bemore easily handled automatically.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a wafer protective sheet according tothe present invention;

FIG. 2 is a fragmentary enlarged view of a wafer protective sheetaccording to the present invention;

FIG. 3 is a perspective illustration showing how wafers are housed in acontainer;

FIG. 4 is a sectional view of an example of wafers housed in acontainer; and

FIG. 5 is a schematic illustration of a method for measuring bendingresistance.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will now be described.

A wafer protective sheet according to an embodiment of the presentinvention is a disk cut out of a sheet material having a large number ofprojected parts 8 and a large number of recessed parts 9 forming latticestripes, as shown in FIGS. 1 and 2. The projected parts 8 and recessedparts 9 are disposed in such a matter as to be positioned atintersections of lattice stripes and alternately each other. The manneras to be positioned at intersections of lattice stripes means a mannerthat when a quadrangle is drawn by connecting four adjacent projectedparts 8 with a line, the recessed part 9 is located in the quadrangle.The quadrangle is not necessarily square, and may be rectangular. Therecess 9 does not necessarily lie at the center of the quadrangle.

The material of the wafer protective sheet 1 has a thickness t (see FIG.2) of 80 to 130 μm, and preferably 80 to 110 μm. A sheet material with athickness of less than 80 μm results in such a light wafer protectivesheet 1 as to flutter, and consequently it becomes difficult to behandled by an automatic machine. Also, if the wafer protective sheet 1has a thickness of less than 80 μm, the synthetic resin sheet needs tohave a high hardness so that the resulting wafer protective sheet 1 doesnot droop when it is handled by an automatic machine. However, a highhardness of the sheet negatively affects the capability of protectingwafers 2 from scratches, and besides makes it difficult to form therecessed parts 8 and the projected parts 9 uniformly. On the other hand,a thickness of more than 130 μm not only increases the costs ofmaterials of the wafer protective sheet 1, but is also disadvantageousin view of disposal (environment).

The height h of the projected parts and recessed parts, defined by thedistance from the bottom of the recessed parts 9 to the top of theprojected parts 8 (see FIG. 2) is preferably set in the range of 20 to50 μm to prevent adhesion. A height of less than 20 μm cannotsufficiently prevent adhesion, and a height of more than 50 μm requiresa larger space for storing the wafer protective sheet 1. The density ofthe projected parts and recessed parts is not particularly limited, butpreferably, the number of projected parts and/or the number of recessedparts is 0.5 to 16 per square centimeter of sheet area of the waferprotective sheet (0.5 to 16/cm²). A density of less than 0.5 is likelyto affect the capability of adhesion prevention, and a density of morethan 16 may not enhance the adhesion prevention.

The area of a single projection or recess is preferably set at 0.3 to3.0 mm² in plan view, from the viewpoint of ease of their formation byheat embossing. Furthermore, the projected parts and recessed parts mustbe smooth without sharp edges so as not to make scratches on wafers.

The wafer protective sheet 1 has a bending resistance of 30 to 80 mm,and preferably 30 to 50 mm. In the present invention, the bendingresistance is defined as the distance X in the horizontal direction ofthe bending formed when a specimen with a width of 15 mm protrudes inthe horizontal direction at a length of 120 mm with its one end held asshown in FIG. 5. A wafer protective sheet with a bending resistance ofless than 30 mm may droop or result in other problems when it is handledby an automatic machine. A wafer protective sheet with a bendingresistance of more than 80 mm is likely to make scratches on wafers 2due to vibration during transportation.

For housing wafers 2 in a container 4, a cushion 3 is disposed on thebottom of the container 4, then the wafer protective sheets 1 and thewafers 2 are alternately stacked, and another cushion 3 is furtherdisposed on the top of the stack, as shown in FIG. 3. Then, thecontainer is closed with a cover 5, as shown in FIG. 4. When the cover 5is put on the container 4, a pressure is applied so that the wafers 2 donot move.

The synthetic resin for forming the wafer protective sheet 1 of thepresent invention is such as to provide an appropriate bendingresistance and appropriate flexibility to the wafer protective sheet 1.Preferably, a high-density polyethylene is used. An antistatic materialor the like may be added to the synthetic resin if necessary, and asynthetic resin compound is thus prepared for the wafer protective sheet1.

Alternatively, another resin having physical properties similar to thehigh-density polyethylene may be used, such as a mixture of polyethyleneand polypropylene or an ethylene-propylene copolymer.

The antistatic material, which is intended to prevent dust or the likefrom landing on the wafer protective sheet 1 and to remove staticelectricity generated by friction between a wafer 2 and the waferprotective sheet 1 effectively, is kneaded to the synthetic resintypically in such an amount as the wafer protective sheet 1 has asurface resistivity of 10¹²Ω or less. Examples of the antistaticmaterial include carbon black, graphite carbon, carbon fiber, metalpowder, metal oxide powder, metal-coated materials, static-free ethylenecopolymer ionomer resins using an alkali metal as an ion source,low-molecular-weight antistatic agents such as surfactants, andpolymeric antistatic agents.

In order to give the wafer protective sheet 1 an antistatic property,the surfaces of the wafer protective sheet 1 can be coated with amaterial having an antistatic property. It is however disadvantageousbecause the coating material may separate to drop off the sheet, andbecause costs increase due to increase of processing steps. It istherefore preferable that the synthetic resin compound be prepared bykneading the antistatic material with the synthetic resin. The syntheticresin compound for preparing the wafer protective sheet 1 preferably hasflexural modulus (JIS K6922-2) of 1.0×10³ to 1.2×10³ MPa. Such amaterial can easily achieve a wafer protective sheet having a bendingresistance of 30 to 80 mm, preferably 30 to 50 mm, at a thickness of 80to 130 μm, preferably 80 to 110 μm. In use of a synthetic resin compoundhaving flexural modulus of less than 1.0×10³ MPa, however, the sheetmust be formed to a large thickness so that the resulting waferprotective sheet 1 has an appropriate bending resistance. This isdisadvantageous in terms of disposal. A synthetic resin having flexuralmodulus of more than 1.2×10³ MPa results in a sheet liable to makescratches on wafers. The flexural modulus, which is a physical propertysimilar to tensile modulus (JIS K7161), is defined by flexural stress ata very low strain specified in JIS.

The wafer protective sheet 1 of the present invention is formed of theabove-described synthetic resin compound.

First, a synthetic resin compound containing such an amount ofantistatic material as achieves a surface resistivity of 10¹²Ω or lessis extruded to form a synthetic resin sheet with a uniform thickness byextrusion method, etc. The thickness of the synthetic resin sheet is setat 80 to 130 μm, preferably 80 to 110 μm.

Then, the resulting synthetic resin sheet is subjected to heatembossing. Specifically, the synthetic resin sheet is heated until it issoftened, and is fed to a pair of rolls having many protrusions atstaggered positions. The projected parts 8 of the front surface, whichcorrespond to recessed parts of the rear surface, are formed atpositions of the synthetic resin sheet where the protrusions press inthe direction from the rear surface to the front surface. Also, therecessed parts 9 of the front surface, which correspond to protrusionsof the rear surface, are formed at positions of the synthetic resinwhere the protrusions press in the direction from the front surface tothe rear surface.

The wafer protective sheet 1 may have flat parts 7, which have not beendeformed, between the projected parts and recessed parts that have beenformed by deforming the synthetic resin sheet, as shown in FIG. 2. Theflat parts help the wafer protective sheets 1 turn flat or substantiallyflat when a pressure is applied to the wafers 2 and wafer protectivesheets 1 housed in a container 4 in the thickness direction of thewafers 2. Thus, the wafer protective sheet can prevent the wafers 2 frombeing scratched more effectively. Preferably, the flat parts 7 occupy60% or more of the total area of the front surface and rear surface ofthe wafer protective sheet 1.

Then, the resulting sheet having the projected parts and recessed partsis cut into a size according to the size of the wafers 2. The waferprotective sheet 1 of the present invention, shown in FIG. 1, is thusobtained. Although the size of the wafer protective sheet 1 is notnecessarily the same as that of the wafers 2, it is preferable that thewafer protective sheet 1 has substantially the same size as the wafersso that a force is not locally placed on the wafers 2.

EXAMPLE

Into 100 parts by weight of each synthetic resin (high densitypolyethylene) shown in Table 1 was added 15 parts by weight of apolymeric antistatic agent (PELESTAT 300 produced by Sanyo ChemicalIndustries, Ltd.) and 1 part by weight of a black pigment. Syntheticresin compounds having flexural modulus shown in Table 1 were thusprepared. The synthetic resin compounds were each formed into asynthetic resin sheet with a thickness t shown in Table 1 by extrusionmethod.

The synthetic resin sheet was heated and subjected to embossing with apair of rolls having many protrusions. The projected parts and therecessed parts were alternately formed in lattice stripes manner on thesheet. The resulting sheet was cut into wafer protective sheets 1 with apress cutting machine of 8 inches in diameter. The height h of theprojected parts and recessed parts was 25 μm; the area of eachprojection or recess was 1.5 mm² in plan view; the density of theprojected parts and recessed parts was 4 per square centimeter; and thepercentage of the flat parts was about 90%.

The following properties of the resulting wafer protective sheets wereevaluated.

Bending Resistance:

Average of data obtained from measurements in case that the frontsurface faced upward and when the rear surface faced upward.

Adhesion Prevention:

The wafer protective sheets were interposed between 8-inch wafers andstored in a container under a pressure of 5 KPa. After storage for 30days, the wafers were taken out and observed whether the waferprotective sheets adhered to the wafers.

◯: No adhesion

X: Adhering

Scratch Prevention:

Wafers and the wafer protective sheets were housed in a container undera pressure of 5 KPa in the same manner as in the evaluation of adhesionprevention, and 5 Hz vibration of 20 mm in amplitude was applied to thecontainer in the direction along the surfaces of the wafers for 1 hour.It was observed with a microscope whether there were scratches on thewafers.

◯: No scratches

Δ: Something like a scratch precursor was found.

X: A scratch was found.

Environmental Property (Disposability):

The thickness of the wafer protective sheet was evaluated as follows:

◯: 110 μm or less

Δ: 110 to 130 μm

X: more than 130 μm

Handleability:

When the wafer protective sheets were taken out one by one from the topof wafer protect sheets, it was thus observed whether the waferprotective sheets left in the stack were misaligned.

◯: No misalignment

X: Misaligned TABLE 1 Flexural Bending Synthetic Modulus ThicknessResistance Adhesion Scratch Environmental Resin (MPa) (t) (μm) (mm)Prevention Prevention Property Handleability Example 1 High-density 1060100 40 ◯ ◯ ◯ ◯ polyethylene 1* Example 2 High-density 1060 80 30 ◯ ◯ ◯ ◯polyethylene 1* Example 3 High-density 1060 130 80 ◯ Δ Δ ◯ polyethylene1* Example 4 High-density 950 115 30 ◯ ◯ Δ ◯ polyethylene 4* Example 5High-density 1260 80 65 ◯ Δ ◯ ◯ polyethylene 2* Comparative High-density1260 70 55 ◯ Δ ◯ X Example 1 polyethylene 2* Comparative High-density730 150 40 ◯ ◯ X ◯ Example 2 polyethylene 3* Comparative High-density1060 70 25 X ◯ ◯ ◯ Example 3 polyethylene 1* Comparative High-density1060 150 70 ◯ Δ X X Example 4 polyethylene 1* Comparative High-density950 100 25 X ◯ ◯ ◯ Example 5 polyethylene 4* Comparative High-density1260 100 90 ◯ X ◯ ◯ Example 6 polyethylene 2*High-density polyethylene 1: KF251A, produced by Japan Polyolefins Co.,Ltd.High-density polyethylene 2: 6000, produced by Tosoh CorporationHigh-density polyethylene 3: AE088L, produced by Japan Polyolefins Co.,Ltd.High-density polyethylene 4: Mixture of high-density polyethylene 2 andhigh-density polyethylene 3 in a ratio of 1:1.

The wafer protective sheets according to the present invention wereprepared from a sheet material with an appropriate thickness whosebending resistance was in an appropriate range after the projected partsand recessed parts had been formed. Consequently, the resulting waferprotective sheet exhibited good characteristics in any of adhesionprevention, scratch prevention, environment, and handling property.

On the other hand, the wafer protective sheet of Comparative Example 1was prepared from a sheet material with thin thickness. Consequently,the wafer protective sheet easily fluttered and was difficult to handle.In order to prevent the wafer protective sheet from drooping when it washandled by an automatic machine, a hard resin was used. Accordingly, theresulting wafer protective sheet was liable to make scratches on thesurfaces of wafers.

The wafer protective sheet of Comparative Example 2 used a sheetmaterial having thick thickness, and accordingly had a problem inenvironment.

The wafer protective sheet of Comparative Example 3 used a sheetmaterial having thin thickness and its bending resistance was so lowthat the projected parts and recessed parts of wafers could not restoreto the original state when the wafers were picked up. Consequently, thewafer protective sheet remained adhering to the wafers and adhesionprevention was thus degraded.

The wafer protective sheet of Comparative Example 4 used a sheetmaterial having thick thickness and its bending resistance was so highas to make scratches on wafers easily. Also, it was disadvantageous interms of environment and handling.

The wafer protective sheet of Comparative Example 5 had such a lowbending resistance as to result in bad adhesion as in ComparativeExample 3.

The wafer protective sheet of Comparative Example 6 had such a highbending resistance that the surfaces of wafers were scratched due to thecontact with the wafer protective sheet.

INDUSTRIAL APPLICABILITY

The wafer protective sheet of the present invention has a large numberof projected parts and a large number of recessed parts. Since theprojected parts and recessed parts are alternately arrayed in such amanner as to be positioned at intersections of lattice stripes, thestiffness of the sheet has no orientation. Since the projected parts areformed on both the front and the rear surface, the resulting sheet hasno distinction between the front surface and the rear surface. Since thebending resistance of the sheet material at a thickness of 80 to 130 μmis set at 30 to 80 mm, the resulting wafer protective sheet can be easyto handle automatically, and can exhibit such a flexibility as theprojected parts and recessed parts turn flat or substantially flat whena pressure is applied in the thickness direction of wafers. Thus, thewafer protective sheet has no possibility of making scratches on wafers.Since the flattened projected parts and recessed parts are restored tothe original state when the container is opened, the adhesion preventioncan be satisfactory.

1. A wafer protective sheet comprising a synthetic resin sheet with athickness of 80 to 130 μm having a large number of projected parts and alarge number of recessed parts on the respective front surface and rearsurface thereof, wherein the large number of projected parts and thelarge number of recessed parts are respectively so disposed that eachpart is positioned at intersections of lattice stripes, and theprojected part and the recessed part are disposed alternately eachother; the sheet has such a wavy cross section that the recessed partsin the rear surface match the projected parts on the front surface andthe projected parts on the rear surface match the recessed parts in thefront surface; and the wafer protective sheet has a bending resistanceof 30 to 80 mm.
 2. The wafer protective sheet according to claim 1,wherein the wafer protective sheet is formed of a synthetic resincompound having a flexural modulus of 1.0×10³ to 1.2×10³ MPa.
 3. Thewafer protective sheet according to claim 1, wherein the height of theprojected parts and recessed parts is 20 to 50 μm.
 4. The waferprotective sheet according to claim 1, wherein the density of theprojected parts and recessed parts, that is, the number of projectedparts per area and/or the number of recessed parts per sheet area, is0.5 to 16 per square centimeter.
 5. The wafer protective sheet accordingto claim 1, wherein the synthetic resin contains an antistatic agent toexhibit an antistatic property.
 6. The wafer protective sheet accordingto claim 5, wherein the wafer protective sheet has a surface resistivityof 10¹²Ω or less.
 7. The wafer protective sheet according to claim 1,wherein the wafer protective sheet has flat parts in addition to theprojected parts and the recessed parts.
 8. The wafer protective sheetaccording to claim 7, wherein the flat parts occupy 60% or more of thetotal area of the front and rear surfaces of the wafer protective sheet.